With environmental problems coming to the fore, accumulator devices have been intensively developed which are used in storage systems for clean energy produced by solar power generation or wind power generation as well as for electric vehicles or hybrid electric vehicles which are replacing gasoline-powered vehicles. Furthermore, in recent years, while on-board systems or devices such as power window systems or IT associated devices have been improved to provide higher performances and functions, there is a need for development of a new accumulator device which provides higher energy density and higher output power properties.
In this context, as an accumulator device conforming to the requirements for higher energy density and higher output power properties, attention has been recently focused on an accumulator device, called “a hybrid capacitor,” for which the principles of accumulation of electric charges in the lithium ion secondary battery and the electric double layer capacitor are combined. As such a hybrid capacitor, an accumulator device was suggested which employed activated carbon to form the positive electrode and a carbonaceous material, capable of occluding and deoccluding lithium ions, to form the negative electrode. The accumulator device was capable of providing a high energy density by allowing the negative electrode to occlude or carry lithium ions (also referred to as “doping”) in advance chemically or electrochemically and thereby lowering the potential of the negative electrode (for example, see Patent Literature 1).
In this context, a hybrid capacitor-type accumulator device was suggested which was to provide a higher energy density (for example, see Patent Literature 2). However, although a high performance was expected, the hybrid capacitor-type accumulator device mainly provided an energy density of 8 to 16 Wh/Kg due to an excessive weight of the current collector, the separator and the electrolytic solution. Furthermore, the accumulator device had a problem that an increase in the energy density caused degradation in service life, thus making it difficult to put the accumulator device to practical use so as to provide a high energy density.